Open the safe's door, and place it into "locked" mode.
Enter six zeros into the number keypad.
Enter the current six-digit combination code.
Enter the new six-digit combination code you wish to use, then enter it again. If the code is invalid, the lock will signal three times, and the old combination will still work.
Open the safe door, and place it into "locked" mode with the door remaining open.
Press the # key, then press the number 1. It will beep and flash once.
Enter the current six-digit code. It will again beep and flash once.
Type in a new six-digit code, after which there will be one beep and one flash.
Type the new six-digit code again, and there will be two beeps and two flashes.
Test the new combination a few times with the door open. If you type in the correct code, you will always get two beeps and two flashes. Making a mistake will cause the lock to revert back to the old code.